In recent years, there has been progressing the exploitation of oil wells and gas wells under various environments, including very deep wells, high temperature and high pressure gas wells, and wells in cold region. Accordingly, there are also aroused the problems of corrosion under high CO2 environments and, for an oil well generating H2S, of sulfide stress corrosion cracking (SSC) caused by H2S. As a result, there is an increased demand of steel pipes having both the 552 MPa or higher yield strength and the high toughness, necessary for OCTG for deep wells enduring above-described severe corrosion environments.
Conventional OCTG materials are 410 Steel or 420 Steel specified by the American Iron and Steel Institute (AISI). Although these grades of steels are relatively inexpensive and achieve 552 MPa or higher yield strength by heat treatment, they do not have satisfactory corrosion resistance and toughness. Furthermore, since these steels contain carbon by about 0.1% or more by weight, they cannot be treated by water-cooling in the manufacturing process, which degrades the production efficiency.
Regarding the martensitic stainless steel having above-described strength, high toughness, and high corrosion resistance, and regarding the method for manufacturing thereof, there are several proposals.
For instance, Patent Literature 1, (Japanese Patent No. 2665009), discloses a steel containing 0.005 to 0.04% C, 12.0 to 17.0% Cr, and 1.5 to 6.0% Ni, by weight, and a method for manufacturing thereof. Although the steel has 784 to 1078 MPa yield strength (proof stress), which is higher than that of general-use 552 MPa grade and 655 MPa grade steels, and although the steel gives favorable corrosion resistance in a 65% nitric acid corrosion test, the steel is not examined for corrosion resistance under high CO2 environments. Patent Literature 2, (Japanese Patent No 2091532), discloses a steel containing 0.15% or less C, 9 to 16.0% Cr, and 0.2 to 2.5% Ni, by weight, and a method for manufacturing thereof. Since, however, the manufacturing of the steel needs controlled rolling, the method has a problem in the efficiency of manufacturing process, and has a limitation on the manufacturing facilities. Patent Literature 3, (Japanese Patent No. 2995524), discloses a steel containing 0.03% or less C, 11 to 17% Cr, and 3.5 to 7.0% Ni, by weight, and a method for manufacturing thereof. Since, however, the steel needs to add 3.5% or more Ni, the steel is not advantageous in economy. Patent Literature 4, (JP-A-2004-115890), (the term “JP-A” referred to herein signifies the “Japanese Patent Laid-Open No.”), discloses a low-Ni steel containing 0.05% or less C, 10 to 12.5% Cr, and 1.5 to 3.0% Ni, by weight, and a method for manufacturing thereof. The steel is, however, limited to 552 MPa grade strength because these materials as shown above rapidly degrade properties such as toughness and SSC resistance if the strength exceeds 552 MPa grade. Patent Literature 5, (JP-A-2004-99964), discloses a low Ni—Nb steel containing 0.02 to 0.05% C, 10 to 12% Cr, 1.5 to 3.0% Ni, and 0.005 to 0.10% Nb, by weight, and a method for manufacturing thereof. The steel is, however, limited to 758 MPa grade of strength.
[Patent Literature 1] Japanese Patent No. 2665009
[Patent Literature 2] Japanese Patent No. 2091532
[Patent Literature 3] Japanese Patent No. 2995524
[Patent Literature 4] JP-A-2004-115890
[Patent Literature 5] JP-A-2004-99964